This invention relates to a variable air/fuel ratio engine control system in combination with Otto-diesel throttling.
For years, automotive engineers have attempted to improve the efficiency of internal combustion automobile engines and present day engines are indeed much more efficient than earlier ones. Heretofore, the application of closed-loop computer control around maximum efficiency has been overlooked since it was thought to be too complicated or too expensive. One way to optimize total engine efficiency is to compute the output torque versus the fuel delivered and then find the point of minimum brake specific fuel consumption (BSFC). The measurement of BSFC has been done in the laboratory for years, but has never been used in a closed-loop system on a car. Although the measurement works well in a laboratory where torque can be measured with a dynamometer, real time torque measurement on a vehicle is expensive and a better alternative is to measure cylinder pressure because it provides so much information.
From measured cylinder pressure, the indicated mean effective pressure (IMEP) can be derived. This parameter is a measure of the average internal cylinder pressure that is applied to the piston to generate torque. It is an accurate torque representative except for the amount of torque lost to internal engine friction. With the IMEP, it is possible to calculate the indicated specific fuel consumption (ISFC). With a measure of ISFC, it is possible to operate an engine very close to its maximum efficiency level at all times. It is also possible to estimate the brake mean effective pressure (BMEP) from the IMEP, assuming some knowledge of friction as a function of engine speed and load. This approach would allow direct control around approximate brake specific fuel consumption (BSFC) for maximum efficiency at all times.
The above approach has not been followed in the past because of emission control regulations. It is generally perceived that the three-way catalyst is the only feasible way to meet emissions regulations. A three-way catalyst, however, requires a stoichiometric air/fuel mixture to achieve the chemical reaction necessary to reduce emissions and, therefore, lean burn has been mostly ignored. So, even though it is recognized that maximum efficiency occurs at lean air/fuel ratios for most speed and load conditions of an internal combustion engine, lean burn has not been exploited because of three-way catalyst requirements. As will become clear below, with the right combination of components and accurate control of these components, a lean burn engine can be designed to pass current emissions regulations.
Some of the components to carry out the present invention have existed for only a short time. Microprocessors are now available which can calculate ISFC of BSFC in real time as well as having the capability to control the large variation in air/fuel ratio and ignition timing necessary to achieve reliable lean burn. Good fuel atomizers have been around for some time, but typically work well only in a specific flow range. High power ignitions have also been known for a long time, but have been very inefficient. As will be discussed below, maximizing combustion efficiency can be coupled with a minimization of total emissions, potentially eliminating the need for a catalyst entirely while passing present emissions standards.
In the past, lean burn control has typically been done by open-loop systems. Because such systems are open-loop, they do not permit new engines to run at peak efficiency because the engine has to be set up to run well at 50,000 miles and beyond. U.S. Pat. No. 4,608,956 discloses such a system, even though it attempts to close the control loop around an exhaust gas sensor to correct for an air/fuel ratio that is off the target air/fuel ratio. This system cannot account for engine wear that might change the appropriate target air/fuel ratio. U.S. Pat. No. 4,825,838 uses the misfire limit as a way to close the loop using vibrations detected by an exhaust gas sensor for feedback. This system does not optimize efficiency because the misfire limit can be well beyond the air/fuel ratio for maximum efficiency. U.S. Pat. No. 4,887,575 discloses a system for determining and controlling the mixture ratio supplied to an internal combustion engine in which the air/fuel mixture ratio is estimated from the maximum internal pressure of an engine cylinder. The system of the `575 patent attempts to maintain substantially a stoichiometric mixture at all times and is merely a way of accurately estimating where that air/fuel ratio occurs.